1. Field of the Invention
The invention resides in the field of optical recording and readout apparatus and more particularly relates to a scanning device employing a semiconductor laser, a diffraction grating, and control means for altering the frequency of the laser to effect the position of the laser beam.
2. Description of the Prior Art
Semiconductor or diode lasers are well known devices employed in information transfer optical devices such as laser printers for computers and compact discs for audio or visual entertainment systems. They offer advantages of small size, low power dissipation, low cost, low noise and long life.
The beam or light spot produced by these lasers must, in order to produce a flow of information, move with respect to the information carrier. In the case of compact or magneto-optical discs, the disc rotates presenting a succession of digitally coded information bits to be read by a reflection pick up system. The laser moves as well to access different portions of the disc along its radius.
Such motion requires motors which need power, introduce mechanical complexity, and generate noise, a highly undesirable trait in the field of audio reproduction. One device that has been suggested to non-mechanically scan the laser beam is an acousto-optic deflector (AOD). These units employ a diffractiion crystal which is modulated by an acoustic wave to produce a modification in the deflectiion of an incident beam as it passes through the crystal. The angle of deflection is proportional to the frequency of the acoustic wave and this effect has been proposed as a means for moving a laser beam in an optical recording device.
U.S. Pat. No. 5,157,650, Ozue et al discloses specific apparatus in which a saw tooth wave is applied to such a modulator for scanning a plurality of laser beams across a moving tape.
The present invention presents an alternative non-mechanical approach to beam scanning which may be combined with the AOD to provide a complete two dimensional electronic scanning format involving no mechanical transport whatsoever.
This is accomplished by taking advantage of the characteristics of laser diodes known as frequency changing. As the operating temperature which is power dependent increases, the output wave length of laser diodes similarly increases. These wavelength increases come in discrete steps as a consequence of the materials of which the diode laser is composed and the fundamental manner in which it operates. See for example: Extension of the Current Continuous Frequency Tuning Range of Ga Al As Laser Diodes by the External Cavity, Shutong el al, SPIE vol. 740; Laser Diode Optics 1987, page 8-11; and Diode Lasers Their Characteristics and Control, Kimball el al, as above pages 41-46.
The present invention proposes to combine this attribute of wavelength variation with power input with the use of a diffraction grating to alter the position of the laser beam emerging from interaction with the grating (transmission or reflection). This technique may be used with a moving substrate on which information is to be recorded or is already encoded or with an AOD to read both dimensions of the carrier.